This application relates to an improved process for leaching copper from a copper sulfide with an aqueous ammonium carbonate solution. This application particularly relates to a method of generating ammonium carbonate, for use in the leaching process, by recovering ammonia and carbon dioxide from the leach liquor formed during the process. This application quite particularly relates to a method for providing additional amounts of carbon dioxide during the recovery of ammonia and carbon dioxide from the leach liquor, in order to make up for losses of carbon dioxide, which are incurred during the treatment of the leach liquor formed during the process.
Processes for leaching copper from a copper sulfide at elevated temperatures which an oxygen-containing gas and an aqueous solution, containing ammonium carbonate as well as ammonia, have been well known. See, for example, U.S. Pat. No. 1,570,858 of Perkins and U.S. Pat. No. 2,727,818 and 2,727,819 of Kenny et al. Generally, in such leaching processes, the leach liquor formed has been heated to distill off gaseous ammonia and carbon dioxide and precipitate copper values from the leach liquor. Frequently, the leach liquor has then been treated in such processes with a strongly alkaline material such as lime. The strongly alkaline material has served to precipitate most of the sulfates, as gypsum, from the leach liquor. This has made it possible to recycle the leach liquor to the copper leaching step of the process and recover the unprecipitated copper values in the leach liquor without undue interference from sulfur compounds in the recycled leach liquor. The strongly alkaline material also has liberated, as a gas, ammonia which has been complexed with the copper in the leach liquor, bound to the precipitated copper values, or dissolved in the leach liquor as ammonium sulfate. In this regard, the use of the strongly alkaline material typically has served to reduce the ammonia content of the precipitated copper values to a practical minimum, so that the ammonia does not unduly interfere with the subsequent electrowinning of the copper. Copper has then been recovered by separating the precipitated copper values from the leach liquor, dissolving the copper values in a strong mineral acid, and electrolytically isolating the copper in relatively pure form.
Frequently in such leaching processes, the gaseous ammonia and carbon dioxide, distilled off during the heating of the leach liquor, have been recovered and utilized to generate ammonium carbonate for use in the leaching of copper. However, before the leach liquor has been heated to distill off ammonia and carbon dioxide, substantial losses of carbon dioxide have inevitably occurred in the course of treating the leach liquor. Such losses of carbon dioxide have mainly taken place in process vessels, containing the leach liquor, that are open to atmosphere, such as apparatuses for separating liquids and solids, holding vessels, and surge tanks. The result of such losses has been that the amounts of carbon dioxide, recovered during the heating of the leach liquor, have generally not been sufficient to generate enough ammonium carbonate to satisfy the needs of the leaching processes. Hence, it has been considered necessary, in carrying out a commercial leaching process with ammonium carbonate, to utilize an external source of carbon dioxide.
Unfortunately, the costs involved in using external sources of carbon dioxide for generating needed amounts of ammonium carbonate have generally been significant. Alternative, less expensive ways have been sought therefore for providing the amounts of carbon dioxide, needed to make up for carbon dioxide losses which inevitably occur when carrying out a leaching process with ammonium carbonate.